Low-rate long-range mode for OFDM wireless LAN

ABSTRACT

A system for implementing an orthogonal frequency division multiplexing scheme and providing an improved range extension. The system includes a transmitter for transmitting data to a receiver. The transmitter includes a symbol mapper for generating a symbol for each of a plurality of subcarriers and a spreading module for spreading out the symbol on each of the plurality of subcarriers by using a direct sequence spread spectrum. The symbol on each of the plurality of subcarriers is spread by multiplying the symbol by predefined length sequences. The receiver includes a de-spreader module for de-spreading the symbols on each of the plurality of subcarriers. The de-spreader module includes a simply correlator receiver for obtaining maximum detection. The correlator produces an output sequence of a same length as an input sequence and the de-spreader module uses a point of maximum correlation on the output sequence to obtain a recovered symbol.

This application claims benefit under 35 U.S.C § 119(e) of provisionalapplication No. 60/624,196, filed on Nov. 3, 2004, the contents of whichis hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to wireless communicationsystems and more particularly to an improvement in the range of awireless LAN device.

2. Description of the Related Art

A wireless communication device in a communication system communicatesdirectly or indirectly with other wireless communication devices. Fordirect/point-to-point communications, the participating wirelesscommunication devices tune their receivers and transmitters to the samechannel(s) and communicate over those channels. For indirect wirelesscommunications, each wireless communication device communicates directlywith an associated base station and/or access point via an assignedchannel. To complete a communication connection between the wirelesscommunication devices, the associated base stations and/or access pointscommunicate with each other directly, via a system controller, thepublic switch telephone network, the Internet, and/or some other widearea network.

Each wireless communication device participating in wirelesscommunications includes a built-in radio transceiver (i.e., receiver andtransmitter) or is coupled to an associated radio transceiver.Typically, the transmitter includes one antenna for transmittingradiofrequency (RF) signals, which are received by one or more antennasof the receiver. When the receiver includes two or more antennas, thereceiver selects one of antennas to receive the incoming RF signals.This type of wireless communication between the transmitter and receiveris known as a single-output-single-input (SISO) communication.

Different wireless devices in a wireless communication system may becomplaint with different standards or different variations of the samestandard. For example, 802.11a an extension of the 802.11 standard,provides up to 54 Mbps in the 5 GHz band and uses an orthogonalfrequency division multiplexing (OFDM) encoding scheme. 802.11b, anotherextension of the 802.11 standard, provides 11 Mbps transmission (with afallback to 5.5, 2 and 1 Mbps) in the 2.4 GHz band. 802.11g, anotherextension of the 802.11 standard, provides 20+Mbps in the 2.4 GHz bandand also uses the OFDM encoding scheme. 802.11n, a new extension of802.11, is being developed to address, among other thins, higherthroughput and compatibility issues. An 802.11a complaint communicationsdevice may reside in the same WLAN as a device that is complaint withanother 802.11 standard. When devices that are complaint with multipleversions of the 802.11 standard are in the same WLAN, the devices thatare complaint with older versions are considered to be legacy devices.To ensure backward compatibility with legacy devices, specificmechanisms must be employed to insure that the legacy devices know whena device that is complaint with a newer version of the standard is usinga wireless channel to avoid a collision.

Currently, most SISO WLANs are IEEE 802.11 complaint. A currentcommunications system provides a range extension on a SISO system bytaking an 802.11a/802.11g signal and cutting the symbol rate.Specifically, the current communications system achieves range extensionby dividing a symbol clock by 24, i.e., the inverse of Super-G, whichdoubles the clock frequency. When the symbol clock is divided, themaximum symbol duration is 96 usec. and the corresponding rate is 250kbps. For example, the current communications system takes an802.11a/802.11g signal that is 16.5 MHz, divides the symbol clock by 24and cuts the signal to 687.5 kHz. When the bandwidth for a signal isreduced, the integrated thermal noise density of the receiver is alsoreduced. Therefore, when the bandwidth is reduced by a factor of 24, thethermal noise floor is decreased by 10*log 10(24). This results in a 16DB “gain” in the sensitivity of the receiver which is equivalent to atleast 3 times improvement in the range of a typical wireless system. Thecost of this implementation, however, is that the data rate is alsodecreased by a factor of 24. Furthermore, since legacy systems in thesame cell as the current communications system may not detect this verynarrow bandwidth, the current communications system does notinteroperate with legacy 802.11a/802.11g systems in the same cell.Specifically, a legacy 802.11a/802.11g device may not detect overlappingBase Service Set (BSS) transmissions from the current system and as suchthe legacy 802.11a/802.11g system will not set its Clear ChannelAssessment (CCA) bits appropriately. Therefore, in dense deployments,such as apartment buildings, network chaos is likely to occur when anactive BSS in the current communications system overlaps with an activelegacy BSS transmission.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a networkdevice implementing an orthogonal frequency division multiplexing schemeand providing an improved range extension. The network device includesreceiving means for receiving data and a symbol mapper for generating asymbol for each of a plurality of subcarriers. The network device alsoincludes a spreading module for spreading out the symbol on each of theplurality of subcarriers by using a direct sequence spread spectrum. Thesymbol on each of the plurality of subcarriers is spread by multiplyingthe symbol by predefined length sequences. The network device furtherincludes transmitting means for transmitting the data to a receiver.

According to another aspect of the invention, there is provided anetwork device for receiving symbols on a plurality of subcarriers andproving improved range extension. The device includes receiving meansfor receiving the plurality of subcarriers. The device further includesa de-spreader module for de-spreading the symbols on each of theplurality of subcarriers. The de-spreader module includes a correlatorreceiver for obtaining maximum detection. The correlator produces anoutput sequence of a same length as an input sequence and thede-spreader module uses a point of maximum correlation on the outputsequence to obtain a recovered symbol.

According to another aspect of the invention, there is provided a methodimplementing an orthogonal frequency division multiplexing scheme fortransmitting data to a receiver and providing improved range extension.The method includes the steps of receiving data for processing andgenerating a symbol for each of a plurality of subcarriers. The methodalso includes the steps of spreading out the symbol on each of theplurality of subcarriers by using a direct sequence spread spectrum,wherein the symbol on each of the plurality of subcarriers is spread bymultiplying the symbol by predefined length sequences; and transmittingthe data to a receiver.

According to another aspect of the invention, there is provided a methodfor receiving symbols on a plurality of subcarriers and providingimproved range extension. The method includes the steps of receiving theplurality of subcarriers and de-spreading the symbols on each of theplurality of subcarriers. The method also includes the steps ofproducing an output sequence of a same length as an input sequence andusing a point of maximum correlation on the output sequence to obtain arecovered symbol.

According to another aspect of the invention, there is provided a systemfor implementing an orthogonal frequency division multiplexing schemeand providing an improved range extension. The system includes atransmitter for transmitting data to a receiver. The transmitterincludes a symbol mapper for generating a symbol for each of a pluralityof subcarriers and a spreading module for spreading out the symbol oneach of the plurality of subcarriers by using a direct sequence spreadspectrum. The symbol on each of the plurality of subcarriers is spreadby multiplying the symbol by predefined length sequences. The receiverincludes a de-spreader module for de-spreading the symbols on each ofthe plurality of subcarriers. The de-spreader module includes a simplycorrelator receiver for obtaining maximum detection. The correlatorproduces an output sequence of a same length as an input sequence andthe de-spreader module uses a point of maximum correlation on the outputsequence to obtain a recovered symbol.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention thattogether with the description serve to explain the principles of theinvention, wherein:

FIG. 1 illustrates a schematic block diagram illustrating acommunication system;

FIG. 2 illustrates a block diagram of a long-range transmitter used inthe present invention;

FIG. 3 illustrates a block diagram of a long-range receiver implementedin the inventive system; and

FIG. 4 illustrates a long-range frame 400 utilized in the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made to the preferred embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 is a schematic block diagram illustrating a communication system10 that includes a plurality of base stations and/or access points12-16, a plurality of wireless communication devices 18-32 and a networkhardware component 34. Wireless communication devices 18-32 may belaptop computers 18 and 26, personal digital assistant hosts 20 and 30,personal computer 24 and 32 and/or cellular telephone 22 and 28. Basestations or access points 12-16 are operably coupled to network hardware34 via local area network connections 36, 38 and 40. Network hardware34, for example a router, a switch, a bridge, a modem, or a systemcontroller, provides a wide area network connection for communicationsystem 10. Each of base stations or access points 12-16 has anassociated antenna or antenna array to communicate with the wirelesscommunication devices in its area. Typically, the wireless communicationdevices register with a particular base station or access point 12-14 toreceive services from communication system 10. Each wirelesscommunication device includes a built-in radio or is coupled to anassociated radio. The radio includes at least one radio frequency (RF)transmitter and at least one RF receiver.

As is known to those skilled in the art, devices implementing both the802.11a and 802.11g standards use an OFDM encoding scheme fortransmitting large amounts of digital data over a radio wave. OFDM worksby spreading a single data stream over a band of sub-carriers, each ofwhich is transmitted in parallel. Specifically, the 802.11a/802.11gstandards specify an OFDM physical layer (PHY) that splits aninformation signal across 52 separate subcarriers to providetransmission of data at a rate of 6, 9, 12, 18, 24, 36, 48, or 54 Mbps.Four of the sub-carriers are pilot sub-carriers that the system uses asa reference to disregard frequency or phase shifts of the signal duringtransmission. The remaining 48 sub-carriers provide separate wirelesspathways for sending the information in a parallel fashion. The 52sub-carriers are modulated using binary or quadrature phase shift keying(BPSK/QPSK), 16 Quadrature Amplitude Modulation (QAM), or 64 QAM.

The present invention uses the OFDM encoding scheme and distributes dataover sub-carriers that are spaced apart at precise frequencies. Thisspacing provides the “orthogonality” which prevents demodulators fromseeing frequencies other than their own. The benefits of OFDM are highspectral efficiency, resiliency to RF interference, and lower multi-pathdistortion. The present invention reuses most of the data path andimplements a more reliable lower rate by applying a Direct SequenceSpread Spectrum (DSSS) to each sub-carrier's stream of QAM sub-symbols.The assumption in OFDM is that each sub-carrier is a flat fadingchannel. Thus, the invention uses a simple matched filter receiver persub-carrier at the receiver with insignificant loss.

FIG. 2 illustrates a block diagram of a long-range transmitter 200 usedin the present invention. RF transmitter 200 includes a scrambler 202, aconvolutional encoder and puncture module 204, a QAM symbol mapper 206,a spreading module 208, and Inverse Fast Fourier Transfer (IFFT) 210, aparallel to serial converter 212 and a cyclic prefix insertion module214. All the bits in the data portion are scrambled by scrambler 202.Scrambling is used to randomize the data, which may contain long stringsof binary data. The data field is then coded by convolutional encoder204 with a coding rate of r=½. Symbol mapper 206 modulates the OFDMsub-carriers by using QAM modulation. Specifically, the data enterssymbol mapper 206 which generates a QAM symbol for each OFDMsub-carrier.

The invention provides spreading gain improvement, wherein after thesymbol are mapped to sub-carriers, spreading module 208 spreads out thesymbol sequence on each of the parallel flat fading channels by using adirect sequence spread spectrum. Therefore, the symbols on each of thesub-carriers are spread out to the full sub-carrier width. According tothe inventive system, each QAM sub-symbol is expanded into a set of Lchips. Frank sequences may be used as the spreading code. According toone embodiment, the symbols are spread using a Constant Amplitude ZeroAuto Correlation (CAZAC) sequence, wherein when a correlation isperformed with itself, a non-zero component is present at only one pointin time. The present invention spreads the symbols using length (L)CAZAC sequences, where L equals to 4, 16 or 64 sequences. As such, onesymbol from symbol mapper 206 is multiplied by L sequences and when L=4,four symbols are produced by the spreading module 208, when L=16, 16symbols are produced by the spreading module 208 and when L=64, 64symbols are produced by spreading module 208.

A spreading sequence when L=16 and (i) is square root of −1 is presentedby the equation:C _(spread,16)=[1+i, −1−i, −1−i, −1−i, 1+i, 1−i, 1+i, −1+i, 1+i, 1+i,−1−i, 1+i, 1+i, 1+i, −1+i, 1+i, 1−i]

When spreading module 208 applies the above spreading sequence, for eachsub-carrier, spreading module 208 outputs a Length 16 sequence. IFFT 210converts the sub-carriers from the frequency domain to the time domain.Parallel to serial converter 212 converts parallel time domain signalsto a plurality of serial time signals. Cyclic prefix insertion module214 introduces the cyclic prefix as a guard interval to eachsub-channel. Therefore, orthogonality can be maintained while bandwidthefficiency is maintained. Transmitter 200 then transmits the OFDMsymbols to a receiver.

FIG. 3 illustrates a block diagram of a long-range receiver 300 used inthe present invention. Receiver 300 receives the OFDM sub-carriers andinstead of making a decision for each symbol, receiver 300 takes a wholescreen of symbols on each of the sub-carriers and runs a correlator oneach of the received sub-carriers. Receiver 300 includes cyclic prefixremoval module 302, Fast Fourier Transfer (FFT) 304, frequency domainmodule 306, de-spreader module 308, QAM symbol demapper 310, parallel toserial converter 312, Viterbi decoder 314, and descrambler 316. Cyclicprefix removal module 302 removes the cyclic prefix inserted bytransmitter 200. Thereafter, FFT 304 converts the serial time domainsignals into frequency signals. Frequency domain module 306 applies aweighting factor on each frequency domain signal. The correlator inde-spreader module 308 despreads the signals that were spread at thetransmitter. The invention allows the use of a simple correlatorreceiver for obtaining maximum detection. The correlator is a matchedfilter and the path of the filter are the spreading sequence timereversed and complex conjugated. As such, the first element of thesequence becomes the last and the last become the first. In the case ofa spreading sequence where L=16, the correlator produces 16 outputs thatcorrespond to the 16 inputs. Thereafter, de-spreader module 308 takesexactly the point of maximum correlation which is exactly the recoveredsymbol. Processing gain in the inventive system of approximately 10*log10(L) is achieved by applying the matched filter per subcarrier sincethe channel decoder processing follows the matched filtering.

Symbol demapper 310 then generates the coded bits from each of thesub-carriers in the OFDM sequence. Parallel to serial converter 312converts the digital time domain signals into a plurality of serial timedomain signals. Viterbi Decoder 314 decodes input symbols to producebinary output symbols. Bits in the data portion are descrambled bydescrambler 318.

The present invention thus allows for the use of the same bandwidth thatis used in legacy systems employing the 802.11a and 802.11g standards.It may also be possible to get a diversity benefit by mappng each of theL chips in a block to a different sub-carrier. Since the equalization isperformed before de-spreading, each received chip may be pulled from adifferent sub-carrier. Although the noise variance on each chip will bedifferent, the present invention provides a frequency diversity benefit.

Furthermore, the data path computational complexity when L=4 requires nomore than one negation operation per transmitted chip beyond processingimplemented in 802.11a/802.11g and no more than one negation operationand one addition per received chip beyond processing implemented in802.11a/802.11g. When L=16 the data path computational complexityrequires no more than two negation operations and two additions pertransmitted chip beyond processing implemented in 802.11a/802.11g and nomore than two negation operations and three additions per received chipbeyond processing implemented in 802.11a/802.11g. Thus, no newmultipliers are required.

As is known to those skilled in the art, each legacy 802.11a/802.11gsystem needs to decode a valid SIGNAL field to determine the length of aframe to set its CCA bit. The legacy SIGNAL field specifies the rate anda length value in bytes which matches the length of the actual frame. Ifadditional information is added to the frame, at the end of the framewhen the legacy receiver attempts to decode the FCS, it detects an errorand discards the frame. FIG. 4 illustrates a long-range frame 400utilized in the present invention. According to the present invention,after the legacy preamble and SIGNAL frame 402, L copies of shorttraining symbols 404 are appended and followed by a proprietary field406. The additional copies of short training symbols 404 allowlong-range receiver 300 to perform carrier detection in extremely lowSNR. Proprietary field 406 includes DSSS-encoded OFDM for long trainingsymbols, SIGNAL and data. The proprietary long training symbols, SIGNALfield and data symbols are transmitted using the inventive DSSSencoding. As such, frame 400 includes information for instructing legacy802.11a/802.11g receivers to ignore field 406. According to theinvention, a legacy system uses the header in preamble 402 to set itsCCA bi,t provided that the actual frame duration does not exceed 5.48msec and the transmissions from the inventive system are above asensitivity threshold. The channel utilization in the current inventionis exactly the same as the channel utilization in a legacy802.11a/802.11g system. Furthermore, there is no need to clock DACs, ADCand logic at lower rates. Additionally, there is no requirement forspecial BSS as the long-range rates are just new rates that can be usedin the same BSS with legacy device. Therefore, compatibility is ensuredby prepending the legacy preamble and SIGNAL fields.

It should be appreciated by one skilled in art, that the presentinvention may be utilized in any device that implements the OFDMencoding scheme. The foregoing description has been directed to specificembodiments of this invention. It will be apparent, however, that othervariations and modifications may be made to the described embodiments,with the attainment of some or all of their advantages. Therefore, it isthe object of the appended claims to cover all such variations andmodifications as come within the true spirit and scope of the invention.

1. A network device implementing an orthogonal frequency divisionmultiplexing scheme and providing an improved range extension, thenetwork device comprising: receiving means for receiving data; a symbolmapper for generating a symbol for each of a plurality of subcarriers,upon receiving the data; a spreading module for spreading out the symbolon each of the plurality of subcarriers by using a direct sequencespread spectrum, wherein the symbol on each of the plurality ofsubcarriers is spread by multiplying the symbol by predefined lengthsequences; and transmitting means for transmitting the data to areceiver.
 2. The network device according to claim 1, further comprisinga plurality of elements for processing the data, the plurality ofelements comprise: a scrambler for scrambling bits in an incoming dataportion; a convolution encoder and puncture module for coding the datawith a coding rate of half, prior to transmitting the data to the symbolmapper; an Inverse Fast Fourier Transfer for converting subcarriers fromthe spreading module from a frequency domain to a time domain; aparallel to serial converter for converting parallel time domain signalsto a plurality of serial time signals; and a cyclic prefix insertionmodule for introducing a cyclic prefix as a guard interval to each ofthe plurality of subcarriers.
 3. The network device according to claim1, wherein the symbol mapper generates a quadrature phase shift symbolfor each of the plurality of subcarriers.
 4. The network deviceaccording to claim 3, wherein the symbol on each of the plurality ofsubcarriers is spread to the full width of the subcarrier.
 5. Thenetwork device according to claim 1, wherein the symbol on each of theplurality of subcarriers is spread using a Constant Amplitude Zero AutoCorrelation sequence, wherein when a correlation is performed withitself, a non zero component is present at only one point in time. 6.The network device according to claim 1, wherein the symbol on each ofthe plurality of subcarriers is spread using length Constant AmplitudeZero Auto Correlation sequences, wherein the length is one of four,sixteen or sixty-four.
 7. The network device according to claim 1,wherein when the length is four the spreading module produces foursymbols for each of the plurality of subcarriers.
 8. The network deviceaccording to claim 1, wherein when the length is sixteen the spreadingmodule produces sixteen symbols for each of the plurality ofsubcarriers.
 9. The network device according to claim 1, wherein whenthe length is sixty-four the spreading module produces sixty-foursymbols for each of the plurality of subcarriers.
 10. A network devicefor receiving symbols on a plurality of subcarriers and providing animproved range extension, the device comprising: receiving means forreceiving the plurality of subcarriers; and a de-spreader module forde-spreading the symbols on each of the plurality of subcarriers,wherein the de-spreader module includes a correlator receiver forobtaining maximum detection, wherein the correlator produces an outputsequence of a same length as an input sequence and the de-spreadermodule uses a point of maximum correlation on the output sequence toobtain a recovered symbol.
 11. The network device according to claim 10,wherein the correlator is a filter and a path of the filter are aspreading sequence time reversed and complex conjugated.
 12. The networkdevice according to claim 10, further comprising a plurality of elementswhich comprise: a cyclic prefix removal module for removing a cyclicprefix from each of the plurality of subcarriers; a Fast FourierTransfer for converting serial time domain signals on each of theplurality of subcarriers to frequency signals; a frequency domain modulefor applying a weighting factor on each frequency domain signal; asymbol demapper for generating coded bits from each of the plurality ofsubcarriers transmitted by the de-spreader module; a parallel to serialconverter for converting time domain signals into a plurality of serialsignals; a Viterbi decoder for decoding the symbols to produce binaryoutput symbols; and a descrambler for descrambling data bits.
 13. Amethod implementing an orthogonal frequency division multiplexing schemefor transmitting data to a receiver and providing an improved rangeextension, the method comprising the steps of: receiving data forprocessing; generating a symbol for each of a plurality of subcarriers;spreading out the symbol on each of the plurality of subcarriers byusing a direct sequence spread spectrum, wherein the symbol on each ofthe plurality of subcarriers is spread by multiplying the symbol bypredefined length sequences; and transmitting the data to a receiver.14. The method according to claim 13, further comprising: scramblingbits in an incoming data portion; coding the data with a coding rate ofhalf prior to the step of generating converting subcarriers from afrequency domain to a time domain, after the step of spreading;converting parallel time domain signals to a plurality of serial timesignals; and introducing a cyclic prefix as a guard interval to each ofthe plurality of subcarriers, prior to the step of transmitting.
 15. Themethod according to claim 13, wherein the step of generating furthercomprises generating a quadrature phase shift symbol for each of theplurality of subcarriers.
 16. The method according to claim 13, whereinthe step of spreading further comprises spreading the symbol on each ofthe plurality of subcarriers to the full width of the subcarrier. 17.The method according to claim 13, wherein the step of spreading furthercomprises spreading the symbol on each of the plurality of subcarriersusing a Constant Amplitude Zero Auto Correlation sequence, wherein whena correlation is performed with itself, a non zero component is presentat only one point in time.
 18. The method according to claim 13, whereinthe step of spreading further comprises spreading the symbol on each ofthe plurality of subcarrier using length Constant Amplitude Zero AutoCorrelation sequences, wherein the length is one of four, sixteen orsixty-four.
 19. The method according to claim 18, wherein the step ofspreading further comprises producing four symbols for each of theplurality of subcarriers when the length is four.
 20. The methodaccording to claim 18, wherein the step of spreading further comprisesproducing sixteen symbols for each of the plurality of subcarriers whenthe length is sixteen.
 21. The method according to claim 18, wherein thestep of spreading further comprises producing sixty-four symbols foreach of the plurality of subcarriers when the length is sixty-four. 22.A method for receiving symbols on a plurality of subcarriers andproviding an improved range extension, the method comprising the stepsof: receiving the plurality of subcarriers; de-spreading the symbols oneach of the plurality of subcarriers; producing an output sequence of asame length as an input sequence; and using a point of maximumcorrelation on the output sequence to obtain a recovered symbol.
 23. Themethod according to claim 22 further comprising the steps of: removing acyclic prefix from each of the plurality of subcarriers; convertingserial time domain signals on each of the plurality of subcarriers tofrequency signals; applying a weighting factor on each frequency domainsignal; generating coded bits from each of the plurality of subcarrierstransmitted by the de-spreader module; converting time domain signalsinto a plurality of serial signals; decoding the symbols to producebinary output symbols; and descrambling data bits.
 24. A system forimplementing an orthogonal frequency division multiplexing scheme andproviding an improved range extension, the system comprising: atransmitter for transmitting data to a receiver, wherein the transmitterincludes a symbol mapper for generating a symbol for each of a pluralityof subcarriers, and a spreading module for spreading out the symbol oneach of the plurality of subcarriers by using a direct sequence spreadspectrum, wherein the symbol on each of the plurality of subcarriers isspread by multiplying the symbol by predefined length sequences; and thereceiver comprises a de-spreader module for de-spreading the symbols oneach of the plurality of subcarriers, wherein the de-spreader moduleincludes a simply correlator receiver for obtaining maximum detection,wherein the correlator produces an output sequence of a same length asan input sequence and the de-spreader module uses a point of maximumcorrelation on the output sequence to obtain a recovered symbol.